Production of ethane for start-up of an LNG train

Abstract

A process is described herein for the production of a selected quantity of ethane as a component of a production inventory of mixed refrigerant for an LNG production plant prior to start-up of the LNG production plant.

What is claimed: 
     
       1. A process for the production of ethane as a component of a production inventory of mixed refrigerant for a liquefied natural gas (LNG) production plant prior to producing liquefied natural gas (LNG) with the liquefied natural gas (LNG) production plant, the liquefied natural gas (LNG) production plant including a liquefaction facility comprising a main cryogenic heat exchanger (MCHE) and a mixed refrigerant (MR) circuit having a mixed refrigerant (MR) compression stage, a propane refrigerant facility which cools a high pressure side of the mixed refrigerant (MR) compression stage and a mixed refrigerant (MR) accumulator in fluid communication with the mixed refrigerant (MR) compression stage, wherein the main cryogenic heat exchanger (MCHE) includes a shell side circuit and a plurality of tube side circuits including a natural gas (NG) tube side circuit, a heavy mixed refrigerant (HMR) tube side circuit, and a light mixed refrigerant (LMR) circuit including a light mixed refrigerant (LMR) tube side circuit and an expansion valve, the expansion valve being in fluid communication with the light mixed refrigerant (LMR) tube side circuit at a cold end of the main cryogenic heat exchanger (MCHE) to feed an expanded fluid stream from the light mixed refrigerant (LMR) tube side circuit into the main cryogenic heat exchanger (MCHE) shell side circuit, the light mixed refrigerant (LMR) tube side circuit being in fluid communication with the mixed refrigerant (MR) accumulator at a warm end of the main cryogenic heat exchanger (MCHE) to receive a light mixed refrigerant (LMR) vapor from the mixed refrigerant (MR) accumulator; and the HMR heavy mixed refrigerant (HMR) tube side circuit is in fluid communication with the mixed refrigerant (MR) accumulator to receive a bottom heavy mixed refrigerant liquid from the mixed refrigerant (MR) accumulator, the process comprising the steps of:
 A) pre-cooling the liquefaction facility to produce a precooled liquefaction facility; 
 B) blanking off the natural gas (NG) tube side circuit and the heavy mixed refrigerant (HMR) tube side circuit; 
 C) directing a bypass stream of dry sweet scrubbed gas through the light mixed refrigerant (LMR) circuit at a first mass flow rate to fill the light mixed refrigerant (LMR) tube side circuit with the bypass stream of dry sweet scrubbed gas; 
 D) after filling the light mixed refrigerant (LMR) tube side circuit, directing the bypass stream to a suction side of the mixed refrigerant (MR) compression stage of the mixed refrigerant (MR) circuit and using the mixed refrigerant (MR) compression stage to compress the bypass stream of dry sweet scrubbed gas and produce a pressurized bypass gas stream which is cooled by the propane refrigerant facility and directed to the mixed refrigerant (MR) accumulator; 
 E) circulating the pressurized bypass gas stream through the light mixed refrigerant (LMR) circuit and the mixed refrigerant (MR) circuit, to progressively cool, partially condense and ethane-enrich the stream by:
 i) feeding vapor from the mixed refrigerant (MR) accumulator through the light mixed refrigerant (LMR) tube side circuit from the warm end of the main cryogenic heat exchanger (MCHE) and subsequently through the expansion valve to cool and partially evaporate any liquid in the stream as it enters the main cryogenic heat exchanger (MCHE) shell side circuit at the cold end of the main cryogenic heat exchanger (MCHE) to form a first fraction rich in nitrogen and methane which is flared from the cold end of the main cryogenic heat exchanger (MCHE), and a second fraction rich in ethane, propane, butane and the heavy hydrocarbons flows through the main cryogenic heat exchanger (MCHE) shell side circuit and cools the bypass stream in the light mixed refrigerant (LMR) tube side circuit by evaporation against the stream in the light mixed refrigerant (LMR) tube side circuit; 
 ii) feeding the evaporated second fraction into the bypass stream being directed to the mixed refrigerant (MR) circuit in step (D) to produce an ethane-enriched stream wherein the ethane-enriched stream is fed to the suction side of the mixed refrigerant (MR) compression stage to produce a compressed ethane-enriched stream; and 
 iii) cooling the compressed ethane enriched stream in the propane refrigerant facility to produce a partially condensed stream containing liquid ethane and storing a condensed portion containing liquid ethane in the mixed refrigerant (MR) accumulator as a condensed heavy mixed refrigerant stream containing liquid ethane and wherein an uncondensed portion of the ethane-enriched stream in the mixed refrigerant (MR) accumulator forms the vapor used in step (E.i); 
 
 F) adjusting the first flow rate of the bypass stream of step (D) to compensate for the mass flow rate of the first fraction being flared in step (E.i); and 
 G) opening the natural gas (NG) tube side circuit and the heavy mixed refrigerant (HMR) tube side circuit to initiate liquefied natural gas (LNG) production when a selected quantity of liquid ethane has accumulated in the mixed refrigerant (MR) accumulator. 
 
     
     
       2. The process of  claim 1  further comprising the step of progressively filling the blanked off heavy mixed refrigerant (HMR) tube side circuit with the condensed heavy mixed refrigerant stream containing liquid ethane. 
     
     
       3. A process for the production of ethane as a component of a production inventory of mixed refrigerant for a liquefied natural gas (LNG) production plant prior to producing liquefied natural gas (LNG) with the liquefied natural gas (LNG) production plant, the liquefied natural gas (LNG) production plant including a liquefaction facility comprising a main cryogenic heat exchanger (MCHE) and a mixed refrigerant (MR) circuit having a mixed refrigerant (MR) compression stage, a propane refrigerant facility which cools a high pressure side of the mixed refrigerant (MR) compression stage and a mixed refrigerant (MR) accumulator in fluid communication with the mixed refrigerant (MR) compression stage, wherein the main cryogenic heat exchanger (MCHE) includes a shell side circuit and a plurality of tube side circuits including a natural gas (NG) tube side circuit, a heavy mixed refrigerant (HMR) tube side circuit, and a light mixed refrigerant (LMR) circuit including a light mixed refrigerant (LMR) tube side circuit and an expansion valve, the expansion valve being in fluid communication with the light mixed refrigerant (LMR) tube side circuit at a cold end of the main cryogenic heat exchanger (MCHE) to feed an expanded fluid stream from the light mixed refrigerant (LMR) tube side circuit into the main cryogenic heat exchanger (MCHE) shell side circuit, the light mixed refrigerant (LMR) tube side circuit being in fluid communication with the mixed refrigerant (MR) accumulator at a warm end of the main cryogenic heat exchanger (MCHE) to receive a light mixed refrigerant vapor from the mixed refrigerant (MR) accumulator; and the heavy mixed refrigerant (HMR) tube side circuit is in fluid communication with the mixed refrigerant (MR) accumulator to receive a bottom heavy mixed refrigerant liquid from the mixed refrigerant (MR) accumulator, the process comprising the steps of:
 A) pre-cooling the liquefaction facility to produce a precooled liquefaction facility, wherein the pre-cooling is achieved by running the propane refrigerant circuit to produce a pre-cooled gas and circulating the pre-cooled gas through the liquefaction facility to precool the liquefaction facility; 
 B) blanking off the natural gas (NG) tube side circuit and the heavy mixed refrigerant (HMR) tube side circuit; 
 C) directing a bypass stream of dry sweet scrubbed gas through the light mixed refrigerant (LMR) circuit at a first mass flow rate to fill the light mixed refrigerant (LMR) tube side circuit with the bypass stream of dry sweet scrubbed gas; 
 D) after filling the light mixed refrigerant (LMR) tube side circuit, directing the bypass stream to a suction side of the mixed refrigerant (MR) compression stage of the mixed refrigerant (MR) circuit and using the mixed refrigerant (MR) compression stage to compress the bypass stream of dry sweet scrubbed gas and produce a pressurized bypass gas stream which is cooled by the propane refrigerant facility and directed to the mixed refrigerant (MR) accumulator; 
 E) circulating the pressurized bypass gas stream through the light mixed refrigerant (LMR) circuit and the mixed refrigerant (MR) circuit, to progressively cool, partially condense and ethane-enrich the stream by:
 i) feeding vapor from the mixed refrigerant (MR) accumulator through the light mixed refrigerant (LMR) tube side circuit from the warm end of the main cryogenic heat exchanger (MCHE) and subsequently through the expansion valve to cool and partially evaporate any liquid in the stream as it enters the main cryogenic heat exchanger (MCHE) shell side circuit at the cold end of the main cryogenic heat exchanger (MCHE) to form a first fraction rich in nitrogen and methane which is flared from the cold end of the main cryogenic heat exchanger (MCHE), and a second fraction rich in ethane, propane, butane and the heavy hydrocarbons flows through the main cryogenic heat exchanger (MCHE) shell side circuit and cools the bypass stream in the light mixed refrigerant (LMR) tube side circuit by evaporation against the stream in the light mixed refrigerant (LMR) tube side circuit; 
 ii) feeding the evaporated second fraction into the bypass stream being directed to the mixed refrigerant (MR) circuit in step (D) to produce an ethane-enriched stream wherein the ethane-enriched stream is fed to the suction side of the mixed refrigerant (MR) compression stage to produce a compressed ethane-enriched stream; and 
 iii) cooling the compressed ethane enriched stream in the propane refrigerant facility to produce a partially condensed stream containing liquid ethane and storing a condensed portion containing liquid ethane in the mixed refrigerant (MR) accumulator as a condensed heavy mixed refrigerant stream containing liquid ethane and wherein an uncondensed portion of the ethane-enriched stream in the mixed refrigerant (MR) accumulator forms the vapor used in step (E.i); and 
 
 F) adjusting the first flow rate of the bypass stream of step (D) to compensate for the mass flow rate of the first fraction being flared in step (E.i). 
 
     
     
       4. The process of  claim 3  wherein the pre-cooled gas is circulated at temperature in the range of −35 to −40° C. 
     
     
       5. The process of  claim 3  wherein the pre-cooled gas is a portion of the bypass stream. 
     
     
       6. The process of  claim 3  wherein the pre-cooled gas is a stream of pre-cooled gas from a fractionation facility or a scrubbing facility. 
     
     
       7. The process of  claim 1  wherein the liquefied natural gas (LNG) production plant includes a scrubbing facility for receiving a dry sweet gas stream and removing hydrocarbons other than methane to produce a dry scrubbed sweet gas stream, and the method includes the steps of:
 (i) pre-cooling the dry sweet gas stream using an intermediate stage of the propane refrigerant facility to produce a pre-cooled dry sweet gas stream; 
 (ii) scrubbing the pre-cooled dry sweet gas stream to produce a bottoms liquid product stream enriched in hydrocarbons heavier than methane and an overhead gaseous product stream; and 
 (iii) cooling the overhead gaseous product stream using the propane refrigerant facility to produce the dry sweet scrubbed gas stream, a portion of which is used as the bypass stream of dry sweet scrubbed gas. 
 
     
     
       8. The process of  claim 7  including the step of splitting the dry sweet scrubbed gas stream into a flared stream having a first mass flow rate and the bypass stream of dry sweet scrubbed gas having a second mass flow rate. 
     
     
       9. The process of  claim 8  wherein the ratio of the first mass flow rate of the flared stream to the second mass flow rate of the bypass stream of dry sweet scrubbed gas is in the range of 5:1 to 2:1. 
     
     
       10. The process of  claim 8  wherein the ratio of the first mass flow rate of the flared stream to the second mass flow rate of the bypass stream of dry sweet scrubbed gas is 4:1 or 3:1. 
     
     
       11. The process of  claim 7  wherein the bottoms liquid product stream is directed to a fractionation facility including a de-ethaniser to produce a recovered ethane stream that is directed to an ethane storage facility. 
     
     
       12. The process of  claim 10  wherein the fractionation facility includes one or both of a de-propaniser to produce a recovered propane stream, and a de-butaniser to produce a recovered butane stream. 
     
     
       13. The process of  claim 7  further comprising the step of directing a circulating stream of the condensed heavy mixed refrigerant stream to an additional cooling stage downstream of the propane refrigeration facility to provide additional cooling to a scrubbing column which facilitates the scrubbing and wherein the condensed heavy mixed refrigerant is evaporated to form an evaporated fraction of the heavy mixed refrigerant. 
     
     
       14. The process of  claim 1  wherein the liquefied natural gas (LNG) production plant is an onshore or floating liquefied natural gas (LNG) production plant. 
     
     
       15. The process of  claim 2  further comprising the step of running the propane refrigerant circuit to produce a pre-cooled gas and circulating the pre-cooled gas through the liquefaction facility to achieve step A). 
     
     
       16. The process of  claim 15  wherein the pre-cooled gas is circulated at temperature in the range of −35 to −40° C. 
     
     
       17. The process of  claim 15  wherein the pre-cooled gas is a portion of the bypass stream of sweet dry scrubbed gas. 
     
     
       18. The process of  claim 15  wherein the pre-cooled gas is a stream of pre-cooled gas from a fractionation facility or a scrubbing facility. 
     
     
       19. The process of  claim 2  wherein the liquefied natural gas (LNG) production plant includes a scrubbing facility for receiving a dry sweet gas stream and removing hydrocarbons other than methane to produce a dry scrubbed sweet gas stream, and the method includes the steps of:
 (i) pre-cooling the dry sweet gas stream using an intermediate stage of the propane refrigerant facility to produce a pre-cooled dry sweet gas stream; 
 (ii) scrubbing the pre-cooled dry sweet gas stream to produce a bottoms liquid product stream enriched in hydrocarbons heavier than methane and an overhead gaseous product stream; and 
 (iii) cooling the overhead gaseous product stream using the propane refrigerant facility to produce a dry sweet scrubbed gas stream, a portion of which is used as the bypass stream of sweet dry scrubbed gas. 
 
     
     
       20. The process of  claim 19  including the step of splitting the dry sweet scrubbed gas stream into a flared stream having a first mass flow rate and the bypass stream of sweet dry scrubbed gas having a second mass flow rate. 
     
     
       21. The process of  claim 20  wherein the ratio of the first mass flow rate of the flared stream to the second mass flow rate of the bypass stream of sweet dry scrubbed gas is in the range of 5:1 to 2:1. 
     
     
       22. The process of  claim 20  wherein the ratio of the first mass flow rate of the flared stream to the second mass flow rate of the bypass stream of sweet dry scrubbed gas is 4:1 or 3:1. 
     
     
       23. The process of  claim 19  wherein the bottoms liquid product stream is directed to a fractionation facility including a de-ethaniser to produce a recovered ethane stream that is directed to an ethane storage facility. 
     
     
       24. The process of  claim 19  wherein the fractionation facility includes one or both of a de-propaniser to produce a recovered propane stream, and a de-butaniser to produce a recovered butane stream. 
     
     
       25. The process of  claim 15  further comprising the step of directing a circulating stream of the condensed heavy mixed refrigerant stream to circulate through an additional cooling stage downstream of the coldest stage of the propane refrigeration circuit. 
     
     
       26. The process  claim 15  wherein the liquefied natural gas (LNG) production plant is an onshore or floating liquefied natural gas (LNG) production plant.